1. Technical Field of the Invention
The present invention relates to interference measurements within timeslots of a TDMA signal, and more particularly, to a method for evaluating a timeslot based upon an amount of interference within the timeslot.
2. Description of Related Art
In order to insure the quality of voice communications over a cellular communications network, the system must avoid cellular calls from being established on disturbed channels containing a level of interference that would degrade the quality of the calls to an unacceptable level. Interference in timeslots can be caused by co-channel interference from another transmitter sending on the same frequency but in another cell. The co-channel interferers are not necessarily slot-synchronized with the TDMA transmitters in this cell. In order to evaluate the disturbed channels, the interference level on selected channels must be measured to enable an estimation of how much the interference level would affect the quality of the call. The interference level on the downlink channels is difficult for the base station to check, unless specified means are included in the standard. The interference level on the uplink channels can be measured at the base station by monitoring received power levels in idle timeslots, i.e., timeslots not used by any mobile connected to that base station. This problem is more fully described in PCT application No. WO 97/31501, which is incorporated herein by reference.
Within analog systems, such as analog AMPS where one RF carrier is dedicated to one mobile station, a straightforward method for making this determination involves measuring and lowpass filtering the received signal strength on idle channels for each analog channel. For TDMA systems, the process is more complicated because interference supervision must be done for each timeslot. This problem is more fully illustrated in FIG. 1 which illustrates an exemplary scenario where an uplink channel frequency is divided into three timeslots 5, 10, 15 of which timeslot one 5 and timeslot two 10 are occupied by a first and a second transmitting mobile stations respectively, connected to the base station transceiver. Timeslot three 15 is idle and is disturbed by a co-channel interferer on the same frequency from a third transmitter in another cell. It will be noted that the interference provided by the co-channel interferer actually occurs over timeslots two 10 and three 15. This is because the second transmitter is not time synchronized/slot synchronized with the timeslots of the receivers for the present base station.
Existing methods of evaluating idle timeslots for e.g., determining whether or not co-channel interference should prevent the assignment of a call to a timeslot involve determining an average interference level for the entire timeslot. Since the unsynchronized interference does not occur over the entire time period of timeslot three 15, a determination may be made that the average interference level in timeslot three is low enough to permit a new call to be set up on the timeslot, even though the first portion of the timeslot is severely disturbed by the co-channel interference. This would present a serious problem for certain types of connections since the channel protection (forward error correction coding) may be weak, and the loss of even a few bits on the air interface may mean loss of the entire slot.
This problem arises because the average interference level determinations are made over the entire timeslot period while significant amounts of interference are only introduced in small portions of the timeslot. The problem is accentuated if those small portions contain critical information, such as bits used for error correction. Thus, prior art system have difficulty handling cases where unsynchronized strong interferers affect only portions of a timeslot since the slotwise interference averages will not give a true picture of the quality of the mobile station to base station voice connection provided by the timeslot at all points within the timeslot. Thus, some way for measuring timeslot interference that reflects the true impact of interference throughout the timeslot is desired.
The present invention overcomes the foregoing and other problems with a method and apparatus for evaluating an idle timeslot within a TDMA signal by detecting interference within the timeslot. The interference is detected by measuring the received signal strength of disturbing signals. Initially, a channel type is selected and a segment or segment size is selected for a plurality of segment average calculations which will be performed throughout the TDMA timeslot. The size of the segment is dependent upon the efficiency of the error correction coding scheme used by the TDMA signal of the selected channel type. Alternatively, the selected segment size may vary within the timeslot depending upon the importance of information contained within particular portions of the timeslot. For example, larger segment sizes may be utilized where non-important information will be transmitted at the beginning of a timeslot, and smaller segments utilized with the more important following information.
Within each of a plurality of segments defined by the selected segment size throughout the timeslot, an average received signal strength is calculated. At least one of the calculated values of the average received signal strength from the plurality of calculated average received signal strengths is selected and input to a lowpass filter. The output of the lowpass filter may be compared to a selected threshold level to enable a determination of whether the idle timeslot interference is low enough to enable a connection utilizing the timeslot. If the filtered average received signal strength output by the lowpass filter exceeds a selected threshold value, an alternative slot must be selected for the connection. If the selected threshold value is not exceeded, the timeslot may be used for a call connection. Alternatively, the interference information may be stored for further analysis e.g., for supporting network optimization.